Pressure-sensitive carbonless copy paper (CCP) has been used for recording business transactions for a number of years. A simple modern form used for recording a transaction comprises plural stacked sheets, where the top sheet has a "transfer" coating on its underside comprising a large number of minute, evenly dispersed microcapsules. Each microcapsule is a spherical polymeric "shell" surrounding a droplet of a liquid "core material" such as a dye precursor solution. The lower sheet has a "developer" coating on its upper surface comprising a dye developer substance. When one makes inscriptive impressions of sufficient force on the upper surface of the top sheet, the microcapsules in the "transfer" coating beneath the impressions are ruptured and release their liquid contents which pass to the underlying "developer" coating. The "developer" coating converts the released dye precursor into a colored dye, thereby generating visible markings on the lower sheet corresponding to the original markings made on the top sheet.
Some CCPs utilize microcapsules containing a colored dye instead of a colorless dye precursor, obviating the need for a "developer" coating. However, use of dyes in CCPs is currently disfavored because it is preferable that the coatings containing microcapsules be colorless. A coating containing a microencapsulated dye retains the color of the dye.
Some CCPs have both the microencapsulated dye precursor and the developer in a single coating, as disclosed, for example, in Dahm et al., U.S. Pat. No. 4,324,817.
Microcapsules for various uses, including for making CCPs, can be formed by a polymerization reaction termed either "interfacial polyaddition" or "interfacial polycondensation," depending upon the polymer chemistry involved. Such processes are termed "interfacial" because the polymerization reaction that forms the microcapsule shell occurs at the interface between a first phase and a second phase, such as between two immiscible liquids. As currently practiced in the art, interfacial polymerization requires at least two separate monomers reactive at an interface to form a polymer: one at least partially soluble in the first phase but insoluble in the second phase, the other at least partially soluble in the second phase but insoluble in the first phase. The typical interfacial polymerization system as used for microencapsulation is an oily, or "hydrophobic", liquid as the first phase and a water-based, or "aqueous", liquid as the second phase.
To make CCP microcapsules, a hydrophobic monomer is added to a hydrophobic liquid which includes a dye or dye precursor material dissolved in a hydrophobic solvent. The resulting mixture, termed the "hydrophobic phase," is added to and emulsified with an aqueous liquid phase to form spherical, substantially uniformly sized microdroplets of the hydrophobic phase suspended in the aqueous phase.
"Emulsification" as used herein denotes the formation of a stable mixture ("emulsion") of at least two immiscible liquids where one liquid is fragmented into a multitude of droplets which are kept in suspension in the other liquid via the presence of a small amount of a substance termed a "stabilizer" or "emulsifier." In an emulsion as described herein, the aqueous liquid phase is termed the "continuous" phase and the totality of microdroplets of hydrophobic liquid held in stable suspension in the continuous phase is termed the "disperse" phase.
The hydrophilic wall-forming monomer should be added to the aqueous continuous phase after forming the emulsion. The hydrophobic and hydrophilic wall-forming monomers react (polymerize) at the phase interface surrounding each microdroplet, forming a rigid spherical shell around each microdroplet. At least one subsequent "curing" step is often preferred to complete the polymerization reaction.
While microencapsulation processes have been known in the prior art for quite some time, not all such processes are amenable for use in making CCP. In any event, the CCP prior art is in general agreement that there are a number of major requirements for microcapsules used in the production of CCP. See, e.g., U.S. Pat. No. 4,299,723 to Dahm et al; U.S. Pat. No. 4,253,682 to Baatz et al. These requirements are listed as follows and referred to herein as the "seven criteria".
First, the microcapsule shell must be impermeable to the core material. As used herein, the "core material" comprises the hydrophobic solution of the one or more compounds to be microencapsulated. Where microcapsules are utilized for CCPs, the core material comprises a solute (such as a dye or dye-precursor compound) dissolved in a hydrophobic solvent and may also include a second hydrophobic solvent or "diluent." Many microencapsulation processes that have been tried in the art for making CCP produce microcapsules that have an unacceptable degree of permeability to either the hydrophobic solvent, the solute, the diluent, or to any combination thereof. Permeability to the solvent or diluent causes drying of the core material inside the microcapsule, resulting in degradation of the copy-forming ability of the CCP. Permeability to the solute causes discoloration of the CCP. On the other hand, the microcapsule shells must break fairly easily upon localized application of a writing or printing force to the paper so that acceptable copies of the writing or printing can be formed using normal writing or printing forces, even if multiple "copy" sheets are used.
Second, microcapsules must be resistant to the relatively small amount of pressure applied to CCP sheets during normal storage and handling. Otherwise, unwanted marks and discoloration would appear on the sheets before use which can seriously degrade the utility of the sheets.
Third, liquid suspensions of microcapsules must be substantially free of agglomerations. Concentrated suspensions of microcapsules having polyurea shells (made from polymerization reactions of aliphatic polyisocyanates and polyamines) are known in the art to have a marked tendency to form agglomerations or clusters of microcapsules during manufacture and subsequent processing. Severe agglomeration can render the microcapsule suspension useless. Even slight agglomeration can form unacceptably granular CCP surfaces. Also, when a suspension of partially agglomerated microcapsules is applied to a paper surface, the clustered microcapsules are more easily ruptured than similar unclustered microcapsules, resulting in unwanted discoloration of the CCP and general degradation of CCP copy-forming ability. Accordingly, suspensions of evenly dispersed microcapsules are highly desired for CCPs.
Fourth, the achievable concentration of microcapsules in a suspension thereof should be as high as possible, up to 50% w/w microcapsules or more, without agglomeration is desirable. The ability to produce concentrated microcapsule suspensions without the need for an energy-intensive concentration or drying step contributes greatly to the requisite economy of CCP production. Also, a high concentration of microcapsules in the transfer coating results in less water having to be removed after application of the transfer coating to the paper surface, which also reduces manufacturing costs.
Fifth, microcapsules used for CCP must be capable of withstanding temperatures of up to about 100.degree. C. for short periods of time. This is because, after application of a microcapsule-containing coating to a paper surface, the coating must be dried, generally by the application of heat. During such heating, the temperature of the coating can easily reach 100.degree. C. for brief periods. The microcapsules must be able to withstand such heating without experiencing significant rupturing, leaking of core material, or other degradation of physical properties.
Sixth, CCPs must be able to withstand storage conditions, even for long periods of time at conditions of relatively high temperature and humidity. The paper must be able to survive such conditions without undergoing discoloration or significant deterioration of copy-forming ability.
Finally, it is preferable that reagents used to produce the suspension of microcapsules, as well as the CCP itself, be relatively safe and not pose a toxicological risk to the worker or consumer.
Achieving a suitably high concentration of nonagglomerated microcapsules in liquid suspension has been a persistent problem in the art. Investigators have tried a number of ways to solve this problem. For example, in Kiritani et al., U.S. Pat. No. 3,796,669, a hydrophobic polyvalent isocyanate and a hydrophobic monomer reactive with the isocyanate are combined with an oily liquid. After adding a hydrophobic polymerization catalyst, the resulting mixture is emulsified into an aqueous phase and appreciably diluted with water to prevent agglomeration of the microcapsules during formation thereof. Similar water dilution steps are also disclosed in U.S. Pat. No. 3,900,669 to Kiritani and U.S. Pat. No. 4,021,595 to Kiritani et al.
Various investigators have tried a number of other processes and polymerization chemistries in an attempt to meet the above-listed requirements, but with limited success. Reference is made to U.S. Pat. No. 4,140,516 to Scher disclosing polyurea microcapsules formed by reaction of an organic isocyanate with a phase transfer catalyst; U.S. Pat. No. 4,119,565 to Baatz et al. disclosing interfacial reactions of polycarbodiimides having terminal isocyanate groups with a hydrophilic "catalyst" such as a tertiary amine; U.S. Pat. No. 4,379,071 to Schnoring et al. in which a polyol is mixed with either phosgene or a polyisocyanate in the hydrophobic phase followed by low-temperature interfacial polymerization with a "chain lengthening agent" such as an amine or a glycol in the hydrophilic phase; U.S. Pat. No. 4,402,856 to Schnoring et al. disclosing temperature-release microcapsules formed from a reaction involving gelatin and a hardening compound; U.S. Pat. No. 4,412,959 to Wegner et al. describing microcapsules prepared by interfacial reaction of a hydrophobic N-containing heterocyclic compound with a hydrophilic compound having at least two terminal OH--, NH--, or SH-- groups; U.S. Pat. No. 4,428,983 to Nehen et al. disclosing an interfacial microencapsulation process wherein both monomer reactants for forming the microcapsule wall are present in either the hydrophobic phase or the hydrophilic phase, where one reactant (generally the amine) is present in a reversibly blocked form that becomes deblocked and therefore reactive with the other reactant (generally an isocyanate compound) when the two phases are combined; U.S. Pat. No. 4,592,957 to Dahm et al. disclosing a "reverse encapsulation" process involving a hydrophilic isocyanate and a hydrophobic polyamine; and U.S. Pat. No. 4,847,152 to Jabs et al. disclosing microcapsules formed via reaction of an aromatic isocyanate with an "isocyanate-reactive" compound such as an amine.
Reference is also made to U.S. Pat. Nos. 4,253,682 to Baatz et al. and No. 4,299,723 to Dahm et al. disclosing interfacial polyaddition reactions involving triketo ring diisocyanates and diamines for forming microcapsules used in making CCPs.
Polyurea microcapsule walls formed via a reaction between a polyisocyanate and a polyamine have become one of the preferred polymerization systems for producing CCP microcapsules. With polyurea, it is possible to produce the desired combination of thin but strong capsule shells. U.S. Pat. No. 4,738,898 to Vivant is one example of such a reaction system in which a "polyisocyanato hydrophobic liquid" is interfacially reacted with a polyamine. The "polyisocyanato hydrophobic liquid" is comprised of a mixture of an aliphatic diisocyanate and an isocyanurate ring trimer of an aliphatic diisocyanate in a ratio of 0.05/1 to 0.70/1. The amount of polyamine is always in excess relative to the "polyisocyanato hydrophobic liquid," rather than in a stoichiometric ratio. Unfortunately, the use of aliphatic diisocyanates poses a serious problem from a toxicological standpoint. We have also found the method is prone to agglomeration during attempts to produce high concentrations of microcapsules when the aliphatic diisocyanate is eliminated. Finally, the microcapsules have permeability to core materials.
While Vivant does not disclose the reason for using excess amine, it is probable that the excess amine increases the formation of exposed amino groups at the interface between the exterior surface of the microcapsule and the aqueous continuous phase. Such exposed amino groups may function somewhat as a stabilizer, conferring some degree of anti-agglomeration stability to the suspension of microcapsules, at least in dilute suspensions. Unfortunately, concentrated suspensions made according to Vivant still exhibit agglomeration. Also, the excess amino groups reduce the degree of crosslinking in the polyurea shell of each microcapsule, rendering the microcapsules more permeable to the core material inside.
Another polyurea process is disclosed in Jabs et al., U.S. Pat. No. 4,428,978, employing as the isocyanate reactant an isocyanurate-modified aliphatic polyisocyanate. The other interfacial polymerization reactant is a "hydrogen active" compound such as an amine. The reaction occurs in the presence of a colloidal stabilizer such as polyvinylalcohol (PVA). The pH of the aqueous continuous phase is adjusted to below pH 7 immediately after adding the amine. Although agglomerate-free suspensions of microcapsules can be produced via this method, the microcapsules unfortunately exhibit unacceptable permeability to the hydrophobic core material with a consequent loss of copy-forming ability when the microcapsules are used in CCPs. The reason for this permeability is unclear, but a possible reason is that excess acid in the aqueous solution causes isocyanate groups to react excessively with the PVA during capsule-wall formation, causing too many PVA molecules to be incorporated into the microcapsule shell. If PVA is incorporated deeply and extensively into the polyurea capsule shell, microchannels extending through the shell wall can be produced due to the incompatibility of the PVA with the polyurea and because of consequent reduction of the amount of crosslinking that would otherwise occur in the capsule shell between isocyanate and amine groups. Such microchannels provide escape routes allowing passage of the core material through the capsule wall, especially during conditions of high heat and humidity, causing a reduction in impermeability.
Another reference pertaining to a polyurea capsule shell is U.S. Pat. No. 4,193,889 to Baatz et al., disclosing microcapsules formed via a polycondensation reaction between an aliphatic polyisocyanate containing at least one biuret group with a "chain-extending agent" such as a polyamine. Unfortunately, this method is only capable of producing relatively low concentrations (about 10% w/w or less) of microcapsules without agglomeration. Also, the microcapsules produced by this method lack sufficient stability for producing satisfactory CCPs.
Yet another reference pertaining to polyurea microcapsules is U.S. Pat. No. 4,668,580 to Dahm et al., disclosing a continuous polyaddition process involving isocyanates that are insoluble in the hydrophobic phase. This method suffers from the disadvantage that at least some degree of solubility of the isocyanate in either the hydrophobic phase or the aqueous phase is required to achieve satisfactory interfacial polymerization. If, as taught in this reference, the isocyanate is insoluble in both the hydrophobic phase and the aqueous phase, interfacial polymerization would be unpredictable in its outcome and would produce microcapsules of a quality unacceptable for use in CCP.
Yet another reference is U.S. Pat. No. 4,785,048 to Chao disclosing interfacial polyaddition of aliphatic or aromatic polyisocyanates or polyisocyanate-epoxy reagents with polyamines to form polyurea microcapsules. High concentrations of colloidal stabilizers and surfactants are used in attempts to inhibit agglomeration and improve impermeability of the capsule wall. Also, both a stabilizer (such as PVA) and a surfactant (such as condensed naphthalene sulfonate) are present during the emulsification step, where the stabilizer is present in a lower concentration than the surfactant. Unfortunately, the microcapsules produced by this process can exhibit high permeability to core material if high amounts of stabilizer are used during capsule manufacture. At the same time, microcapsules can agglomerate during heat curing unless sufficient stabilizer is included.
Hence, there remains a need for a process that will yield agglomerate-free suspensions of polyurea microcapsules, even at high concentrations of microcapsules such as 50 and 60% w/w, that do not undergo agglomeration even when subjected to high temperature after formation of the microcapsules.
There also remains a need for such a process yielding polyurea microcapsules suitable for CCP use, where the microcapsules exhibit a high degree of impermeability to the hydrophobic "core material" such as a organic solution of CCP dye or dye-precursor, even during extended storage at elevated temperature and humidity.
There is also a need for CCP polyurea microcapsules that break easily upon application of a normal writing or printing force to paper coated with a suspension of the microcapsules, but have a high resistance to breaking during normal handling and storage conditions.
There is also a need for a process for manufacturing polyurea microcapsules that does not require the use of toxic aliphatic diisocyanates as an isocyanate reactant.